The invention relates to dispersing gas into molten metal. More particularly, the invention relates to a device, such as a rotary degasser, having an impeller that efficiently mixes gas into molten metal and efficiency displaces the molten metal/gas mixture.
As used herein, the term xe2x80x9cmolten metalxe2x80x9d means any metal in liquid form, such as aluminum, copper, iron, zinc and alloys thereof, which is amenable to gas purification or that otherwise has gas mixed with it. The term xe2x80x9cgasxe2x80x9d means any gas or combination of gases, including argon, nitrogen, chlorine, fluorine, freon, and helium, that are mixed with molten metal.
In the course of processing molten metals it is sometimes necessary to treat the molten metal with gas. For example, it is customary to introduce gases such as nitrogen and argon into molten aluminum and molten aluminum alloys in order to remove undesirable constituents such as hydrogen gas and non-metallic inclusions. Chlorine gas is introduced into molten aluminum and molten aluminum alloys to remove alkali metals, such as magnesium. The gases added to the molten metal chemically react with the undesired constituents to convert them to a form (such as a precipitate or a dross) that separates or can be separated from the molten metal. In order to improve efficiency the gas should be dispersed (or mixed) throughout the molten metal as thoroughly as possible. The more thorough the mixing the greater the number of gas molecules contacting the undesirable constituents contained in the molten metal. Efficiency is related to, among other things, (1) the size and quantity of the gas bubbles, and (2) how thoroughly the bubbles are mixed with the molten metal throughout the vessel containing the molten metal.
It is known to introduce gases into molten metal by injection through stationary members such as lances or porous diffusers. Such techniques suffer from the drawback that there is often inadequate dispersion of the gas throughout the molten metal. In order to improve the dispersion of the gas throughout the molten metal, it is known to stir the molten metal while simultaneously introducing gas, or to convey the molten metal past the source of gas injection. Some devices that stir the molten metal while simultaneously introducing gas are called rotary degassers. Examples of rotary degassers are shown in U.S. Pat. No. 4,898,367 entitled xe2x80x9cDispersing Gas Into Molten Metalxe2x80x9d and U.S. Pat. No. 5,678,807 entitled xe2x80x9cRotary Degassers,xe2x80x9d the disclosures of which are incorporated herein by reference.
Devices that convey molten metal past a gas source while simultaneously injecting gas into the molten metal include pumps having a gas-injection, or gas-release, device. Such a pump generates a molten metal stream through a confined space such as a pump discharge or a metal-transfer conduit connected to the discharge. Gas is then released into the molten metal stream while (1) the stream is in the confined space, or (2) as the stream leaves the confined space.
There are several problems associated with the prior art devices that make them relatively inefficient. Inefficient in this sense means that the known devices do not efficiently disperse gas into the molten metal bath. Therefore, the impurities in the molten metal are not adequately removed and/or an inordinate amount of gas is used to remove the impurities. The inefficiency of the prior art devices is a function of, among other things, their (1) inability to create small gas bubbles to mix with the molten metal, and (2) displace the gas bubbles and/or the molten metal/gas mixture throughout the vessel containing the molten metal. With the prior art devices (other than certain of the previously-described pumps), gas released into the bath tends to rise vertically through the bath to the surface, and the gas has little or no interaction with the molten metal in the vessel relatively distant from the gas-release device. The molten metal/gas mixture is not sufficiently displaced throughout the entire bath. Therefore, to the extent gas is mixed with the molten metal, it is generally mixed only with the molten metal immediately surrounding the prior art device.
It is also known to inject degassing flux through an opening into the molten metal, which again, results in the flux mixing with only the molten metal near where it is released.
The present invention provides an improved device and method for dispersing gas within molten metal. The invention is used in a vessel containing a molten metal bath, and the invention preferably includes (1) a shaft (sometimes referred to herein as an impeller shaft) having a first end, a second end and a passage for transferring gas, (2) an impeller (also referred to as a rotor) having a connector, a top surface, a lower surface, a gas-release opening, and a plurality of cavities open to the lower surface, and (3) a drive source for rotating the shaft and the impeller. The first end of the shaft is connected to the drive source and the second end is connected to the connector of the impeller. The impeller is designed to displace a large volume of molten metal thereby efficiently circulating the molten metal within the vessel. The impeller is preferably rectangular (and most preferably square) in plan view, has four sides, a top surface and a lower surface, and includes a plurality of cavities open to the lower surface of the impeller. Preferably, there are four cavities, one being centered on each side of the impeller. The connector is preferably located in the top surface of the impeller and connects the impeller to the second end of the shaft. Most preferably the connector is a threaded bore extending from the top surface to the lower surface of the impeller thereby forming an opening in the top surface and the lower surface. The upper portion of the bore threadingly receives the second end of the shaft. The gas-release opening may be the opening in the lower surface of the impeller formed by the bore. The passage in the shaft preferably terminates at the second end at an opening. The second-end of the shaft, and the preferred embodiment of the opening therein, may be flush with or extend beyond the opening in the lower surface of the impeller. The gas-release opening may be the opening in the second end of the shaft, which is preferred.
The drive source rotates the shaft and the impeller. A gas source is preferably connected to the first end of the shaft and gas is released into the passage. The gas passes through the passage and is released through the gas-release opening(s). At least part of the gas enters the cavities where it is mixed with the molten metal entering the cavities. The molten metal/gas mixture is displaced radially by the impeller as it rotates.
Optionally, the invention can utilize a dual-flow (or mixed-flow) impeller. Dual-flow means that the impeller both directs molten metal downward into the molten metal bath and outward away from the impeller. The dual-flow impeller-of the present invention preferably has a plurality of vanes wherein each vane preferably comprises: (1) a first surface to direct molten metal downward into the molten metal bath, and (2) a second surface to direct molten metal outward from the impeller. The first surface is preferably positioned on a horizontally-oriented projection that includes a leading edge, an upper surface and a lower surface. The first surface is preferably-an angled wall formed in the lower surface of the horizontally-oriented projection near the leading edge. The second surface is preferably a vertical face beneath the horizontally-oriented projection that directs the molten metal outward from the impeller. Each vane includes a trailing side (opposite the first surface and second surface) that preferably includes a recess that improves the efficiency of the rotor by allowing more molten metal to enter the pump chamber.
Further, the invention may include a tri-flow rotor that (1) directs molten metal downward into the molten metal bath, (2) directs molten metal upward from the lower of the molten metal bath, and (3) directs molten metal outward from the impeller.
Another aspect of the present invention are impellers that can be used with a degassing device according to the invention.